(1) Field of the Invention
This invention relates to a radiographic apparatus for use as an X-ray fluoroscopic apparatus or X-ray CT apparatus, and more particularly to a technique for removing scattered radiation.
(2) Description of the Related Art
Conventionally, in order to prevent scattered X-rays (hereinafter called simply “scattered rays”) transmitted through a subject or patient from entering an X-ray detector, a medical X-ray fluoroscopic apparatus or X-ray CT (computed tomography) uses a grid (scattered radiation removing device) for removing the scattered rays. However, even if the grid is used, a false image is produced by the scattered rays passing through the grid, and a false image by absorbing foil strips constituting the grid. Particularly where a flat panel (two-dimensional) X-ray detector (FPD: Flat Panel Detector) with detecting elements arranged in rows and columns (two-dimensional matrix form) is used as the X-ray detector, a false image such as a moire pattern is produced due to a difference between the spacing of the absorbing foil strips of the grid and the pixel spacing of the FPD, besides the false image by the scattered rays. In order to reduce such false images, a false image correction is needed. In order not to produce such a moire pattern, a synchronous grid has been proposed recently, which grid has absorbing foil strips arranged parallel to either the rows or the columns of the detecting elements, and in a number corresponding to an integral multiple of the pixel spacing of the FPD, and a correction method for use of this grid is also needed (see Japanese Unexamined Patent Publication No. 2002-257939, for example).
By way of correcting moire patterns, a method of image processing which includes smoothing, for example, is carried out nowadays. When false image correction is done to excess, the resolution of direct X-rays (hereinafter called simply “direct rays”) also tends to lower. Therefore, an attempt to reduce false images reliably through image processing will lower the resolution of direct rays, resulting in less clear patient images. Conversely, when greater importance is placed on the resolution of direct rays to obtain clear patient images, the false images will not be reduced through image processing, which constitutes what is called a trade-off between image processing and clearness. Thus, a perfect false image processing is difficult. Regarding the correction of the scattered rays remaining despite use of a grid, various methods have been proposed but these have disadvantages such as involving a time-consuming correcting arithmetic operation.
In connection with the correction method for use of a synchronous grid, Applicant herein has already proposed a method in which correction is carried out with respect to pixels shielded from direct rays by the absorbing foil strips, a distribution of scattered rays having passed through the grid is derived from the columns or rows of the shielded pixels, and signals of the other pixels are corrected based on the distribution. It has been proposed in the above method to set the distance between the grid and X-ray detector to an integral multiple of the height of the absorbing foil strips, and to set the position of the grid and the shape of the absorbing foil strips such that shadows of the absorbing foil strips fall only on certain pixel columns or pixel rows despite changes in the positions of a radiation emitting device such as an X-ray tube, the grid and the X-ray detector.
Further, Applicant herein has also proposed a radiographic apparatus having a function to process false images and acquire an image only of direct rays (see Japanese Unexamined Patent Publication No. 2009-172184, for example). This proposed radiographic apparatus (X-ray imaging apparatus in an embodiment) obtains, as false image processing parameters, before X-ray imaging, direct ray transmittances which are ratios between direct ray intensity before transmission through a grid and direct ray intensities after transmission through the grid, and rates of change relating to transmission scattered ray intensities which are scattered ray intensities after transmission through the grid. Based on a false image processing algorithm using the above parameters, an image only of direct rays can be acquired without false images resulting from the grid.
However, 1. grids other than synchronous grids are in wide use, and the above methods proposed by Applicant herein cannot be applied to such other grids.
2. Even where a synchronous grid is used, the above methods do not take into consideration the influence of a displacement due to deformation of the absorbing foil strips forming the grid, or shifting of position and direction of the entire grid caused by the arrangement of the absorbing foil strips not exactly parallel to either the rows or the columns of the detector.
3. Even if there is no displacement of the absorbing foil strips or the entire grid, the above methods do not take into consideration the influence of the distance between the X-ray tube (radiation emitting device) and FPD (radiation detecting device) deviating from a convergence distance (also called “standard SID”) of the grid (scattered radiation removing device).
4. The method of Japanese Unexamined Patent Publication No. 2009-172184 proposed by Applicant herein gives an equation concerning an nth pixel, Gn=Pn·Cpn+Scn, where Gn is an actual measurement radiation intensity (actual measurement intensity in an embodiment) obtained by actual measurement, Pn is an estimated direct ray intensity which is a direct radiation intensity before transmission through the scattered radiation removing device (grid in the embodiment), Cpn is a direct ray transmittance, and Scn is a transmission scattered ray intensity which is a scattered radiation intensity after transmission through the scattered radiation removing device (grid in the embodiment). Through this equation, the estimated direct ray intensity Pn which is data of only direct rays (to be determined finally) is derived from the transmission scattered ray intensity Scn and direct ray transmittance Cpn.
However, in positions (e.g. peripheral positions of the FPD) remote from a standard position lying on a normal extending from the focus of the X-ray tube to the FPD, the shadows of the absorbing foil strips have larger widths than the shadows of the absorbing foil strips adjacent the standard position, reducing the direct ray transmittance Cp. Further, because of a design error such as a displacement due to deformation of the absorbing foil strips, an actual direct ray transmittance Cpn becomes smaller than a design direct ray transmittance Cpn When an estimated direct ray intensity Pn is derived from the actual measurement radiation intensity G using the above-noted equation, the direct ray transmittance Cpn has a value of one or less, and the transmission scattered ray intensity Scn has a value of one or more. Naturally, therefore, the estimated direct ray intensity Pn has a larger value than the actual measurement radiation intensity G. This is clear from the fact that the actual measurement radiation intensity is a value obtained after transmission through the grid. Therefore, when the estimated direct ray intensity Pn is obtained, variations (what is called deviations) due to statistical fluctuations of the actual measurement radiation intensity G will also become enlarged. When the actual direct ray transmittance Cpn is smaller than direct ray transmittances at other pixels from the above-noted causes (i.e. position remote from the standard position, and design error), the enlargement ratio also becomes larger than at the other pixels, and the variation due to a statistical error is also enlarged by a greater extent than variations at the other pixels, to be conspicuous on the image.
5. When a ratio concerning variations of the direct ray transmittance Cpn in the direction along the absorbing foil strips is a rate of change of the direct ray transmittance, the distortion of the absorbing foil strips and the like cause the direct ray transmittance Cpn to vary sharply between the pixels in the direction along the absorbing foil strips. When the rate of change is large, an estimation error takes place with the direct ray transmittance Cpn. In order to eliminate the variations due to the statistical error of the direct ray transmittance Cpn along the direction of the absorbing foil strips, an average of direct ray transmittances Cpn of a predetermined number of pixels (e.g. 20 pixels or 30 pixels) is usually calculated, and an estimated direct ray intensity Pn is determined using the average. Therefore, when the rate of change of the direct ray transmittance is large due to distortion of the absorbing foil strips and the like, direct ray transmittances Cpn with extremely large values or extremely small values will falsify the average itself. Images with the extremely large values or extremely small values will appear locally in each pixel area formed of the predetermined number of pixels used for obtaining the average. It is to be noted here that the rate of change of the direct ray transmittance is different from the foregoing rate of change about transmission scattered ray intensity.
Problems 1-3 can be solved by the technique disclosed in Japanese Unexamined Patent Publication No. 2009-172184. Problems 4 and 5 cannot be solved where the equation (Gn=Cpn+Scn) in Japanese Unexamined Patent Publication No. 2009-172184 is used.